Recent studies have identified a novel nucleolar protein, Bop 1, as an essential component of the mammalian rRNA processing machinery. Inducible expression of a dominant negative inhibitor of Bop 1 leads to' blockade of 28S and 5.8S rRNA maturation, and unexpectedly, a powerful cell cycle arrest at Gi. Remarkably, this cell cycle arrest is dependent on the activities of p53 in mammalian cells and is not observed with a similar rRNA processing block in yeast. Thus, these observations indicate the presence of a mammalian-specific, p53-mediated surveillance mechanism that monitors nucleolar pre-ribosome assembly as a novel cell cycle checkpoint. These exciting findings provide a new perspective on understanding the role of ribosome biogenesis in cell cycle control and may explain the action of widely practiced but poorly understood chemotherapeutic agents that block RNA synthesis. It is possible to envisage that this novel mechanism of surveillance may also recognize other forms of nucleolar dysfunction, in addition to rRNA processing errors, as "nucleolar stress." Thus, nucleolar stress may be subject to cell cycle checkpoint control, analogous to DNA damage and replicative errors. In this proposal, several questions of broad significance regarding this heretofore unknown cell cycle regulatory mechanism will be addressed. First, is p53-mediated surveillance a general mechanism of detecting nucleolar stress and do errors in pre-ribosome assembly always lead to a cell cycle block in Gi? Second, what is the mechanism of p53 activation upon induction of nucleolar stress? Third, the hypothesis that p53 or Mdm2 may detect nucleolar dysfunction through interaction with nucleolar ribonucleoprotein complexes will be tested. Fourth, proteins that interact with a region of Bop 1 critical for generating a cell cycle arrest signal will be identified and characterized. Through these studies we hope to elucidate the mechanism by which nucleolar surveillance functions as a novel cell cycle checkpoint.